On-chip Synthesis of Nanoporous Silver Microstructures (np-AgMSs) as Mesoporous Materials for SERS Substrate Application

Sensitivity and reproducibility of SERS detection were strongly related to the quality of hotspots. Mesoporous materials provide high quality hotspots compared to normal nano colloids because the size of hotspots is constant... [ view full abstract ]

Sensitivity and reproducibility of SERS detection were strongly related to the quality of hotspots. Mesoporous materials provide high quality hotspots compared to normal nano colloids because the size of hotspots is constant unlike dynamic hotspots that generated from electrostatic interaction of nono colloids. In this work nanoporous Ag microstructures (np-AgMSs) as mesoporous materials were synthesized using a template method in a microfluidic system. Using miniaturized flow-based system, mass and heat transfer are improved. Consequently, the growth of particles was fast and more reproducible compared to batch synthesis. A flow focusing microfluidic device composing of 3 inlets and 1 outlet was used. The synthesis started with fabrication of AgCl templates. A soluble complex of [Ag(NH₃)₂]⁺was pre-prepared off-chip by mixing 0.1 M AgNO₃ with 0.15 M NH₄OH. The Ag(NH₃)₂⁺ solution was pumped through the middle inlet or central flow, while 0.2 M NaCl solution was delivered through the side inlets or side flows (Fig.1). When both solutions were mixed, Cl^-induced precipitation of [Ag(NH₃)₂]⁺ was occurred. AgCl particles were instantly formed in the middle of microchannel (Fig.2). Effect of reactant concentration was studied by simply changing input reactant flow rates. The results showed that the concentration of reactants directly affected the morphology of AgCl templates. By changing the input flow rate of reactants systematically, four different morphologies of fabricated AgCl including cubic, tetrahedral, tripod and tetrapod were observed (Fig. 3). Particle size was in the range of 3-6 µm which was easily observed under microscope. The obtained AgCl templates were subsequently reduced using NaBH₄ to form np-AgMSs. The AgCl templates and np-AgMSs were  characterized using SEM-EDS. After reduction with NaBH₄, structures of the parent AgCl could be retained (Fig. 4). SEM results showed that the interconnected grain size was found to be in the range of 40-65 nm. Finally, the np-AgMSs were tested as SERS substrates using p-aminothiophenol. It was found that each np-AgMS can be used as a highly sensitive single particle SERS substrate for the detection of p-aminothophenol.